IncrementalSAXSource_Filter
IncrementalSAXSource_Filter implements IncrementalSAXSource, using a
standard SAX2 event source as its input and parcelling out those
events gradually in reponse to deliverMoreNodes() requests.

class

IncrementalSAXSource_Xerces
IncrementalSAXSource_Xerces takes advantage of the fact that Xerces1
incremental mode is already a coroutine of sorts, and just wraps our
IncrementalSAXSource API around it.